Wireless equipment usually undergoes many kinds of tests in order to ensure sufficient performance. Some tests are mandated by standards, while others are performed as part of product development and verification. A particular class of tests is that where the over-the-air performance of the communication between one or several wireless transmitter and receivers is tested. The purpose of such a test could be to ensure that handsets are adequately designed and work well in the presence of a human being, which is important for operators when determining whether to subsidize mobile handsets from different vendors, or it could be to evaluate performance under realistic load and channel conditions which would be difficult to achieve at reasonable cost using cabled equipment.
Several different types of over-the-air testing exist. Some examples of the most common ones are described in the following paragraphs.
Reverberation Chamber Testing
Here the transmitting and receiving antennas are placed in a reverberation (or scattered field) chamber, which in its simplest form is a metallic box that gives rise to numerous reflections causing an ideally isotropic multipath distribution. The purpose of reverberation chamber testing is to test radiated performance of mobile stations. Specially designed reverberation chambers, so called Fading Boxes (FBoxes), are used to generate a fading radio environment suitable for system testing of a Radio Access Network. A reverberation chamber is schematically illustrated in FIG. 1.
The mobile station 120 under test, commonly referred to as the device under test (DUT), is either placed physically inside the reverberation chamber 110 or connected via cables to test antennas placed inside the chamber 110. In FIG. 1, the transmitting and receiving antennas 130 of the radio base station (RBS) are placed inside the chamber 110 and are connected to the RBS antenna ports on the outside of the chamber. A rotating stirrer 140 is introduced in the chamber 110 in order to achieve different boundary conditions to obtain fading conditions that vary in time, and thereby simulate that the mobile stations are moving around. A line of sight shield 150, may also be introduced in the chamber to affect e.g. fading or delay conditions.
The reverberation chamber 110 generates a radio environment where the received signals are faded according to a Rayleigh distribution and with a delay spread that is proportional to the size of the reverberation chamber. The radio signals from the mobile stations 120 in such a reverberation chamber are received by the RBS antennas 130 with a rather small delay spread due to the short propagation paths in the reverberation chamber. FIG. 2 shows an example of a channel model, characterized by a distribution of the signal power on a number of taps spread in time, so called delay taps. Each tap is faded and has a specified time distance to other taps as well as a specified power relative to other taps. The reverberation chamber 110 illustrated in FIG. 1 will by design generate a channel having a certain channel impulse response that may be similar to some particular channel model. For instance, the ITU (International Telecommunication Union) have described a set of channel models ranging from low to high time dispersion conditions, to be used for evaluating performance under the expected channel conditions in wireless cellular systems. A particular fading box could be designed to give channel conditions resembling one such model, but not the full range of models.
Anechoic Chamber Testing
In contrast to the reverberation chamber, the inside walls of the anechoic chamber are covered with absorbing material in order to reduce reflections to a minimum. This allows for an ideally deterministic radio environment to be set up, either with a single transmitter and/or receiver forming a line-of-sight link, or using multiple transmitters and/or receivers to create a deterministic multipath distribution.
Field Measurements
This is perhaps the most straight-forward method for testing. In field measurements, also referred to as drive tests, the products are tested in the real environment where they are to be used, or in a semi-controlled environment having characteristics similar to the real environment.
The large potential gains promised by the use of multiple transmit and/or receive antennas, also referred to as Multiple Input Multiple Output (MIMO) techniques, has resulted in such technology becoming an integral part of many wireless standards, such as LTE, WCDMA, WIMAX, etc. It is therefore expected that a multitude of equipment with multiple antennas will enter the market. One challenge is that the performance of MIMO products in particular depends both on the ability of the wireless channel to support multiple orthogonal communication channels, i.e. the channels “richness”, and on how well the antenna configurations at both ends of the link allow utilizing the “richness” of the wireless medium to support diversity or multi-stream transmission.
The over-the-air performance of a certain MIMO-capable mobile station can therefore be expected to depend not only on the design of the mobile station itself, but also on both the wireless channel conditions and the antenna configuration at the other end of the wireless link. The performance will thus for example depend on whether spatial diversity or polarization diversity is used, or it may depend on the spatial separation of antennas in a space diversity setup.
As the performance of especially MIMO equipment is expected to depend both on the wireless channel as well as the antenna configurations at both ends of the link, the number of test cases may become large. The need for testing against different antenna configurations at the other end of the link may require physically shifting antennas or switching between multiple pre-installed antennas, which can become very complex and time-consuming.
Furthermore, in the case of reverberation chamber testing, it would be necessary to have reverberation chambers of different sizes in order to create reverberation chamber test environments with different delay spreads, since the delay spread of a reverberation chamber is proportional to the size of the reverberation chamber. A change of delay spread during testing would thus also require complex and time consuming reconfigurations of the test environment. Besides, the size of a reverberation chamber intended to simulate a delay spread that corresponds to a channel model representing for example a rural area environment would have a too large foot print to be placed in a test lab.